A sulfurous acid gas is generally contained in a combustion exhaust gas (hereinafter also referred to as “exhaust smoke”) emitted from a boiler or the like of a thermal power plant, and in some cases, mercury may be contained at a high concentration according to the kind of fossil fuel (in particular, coal) burned. When they are emitted into the environment, they become harmful substances causing a health hazard, and so it is necessary to remove such harmful substances before the combustion exhaust gas is emitted in the air. Removal of such sulfurous acid gas has heretofore been required by the emission regulation, and an international effort on prevention of mercury pollution has been strengthened in addition to this regulation in recent years.
Methods for removing the sulfurous acid gas (SO2) in the combustion exhaust gas include a wet method of removing it by being absorbed in an absorbing liquid and a dry method of removing it by being adsorbed on an adsorbent, and as for each of such methods various methods are known. When a large amount of a combustion exhaust gas containing a sulfurous acid gas at a high concentration is treated, however, the wet method which is advantageous in the treatment cost is generally adopted.
Mercury in the combustion exhaust gas includes divalent mercury (Hg2+) present in the form of a divalent mercury compound by being oxidized by an oxidation catalyst or the like in a combustion furnace or an exhaust smoke denitrification apparatus and zero-valent mercury (Hg(0)) present in the form of metal mercury of a simple substance (zero valence). Hg2+ is almost removed by a wet method exhaust smoke desulfurization apparatus. However, the removal efficiency of the zero-valent mercury is low because its solubility in the absorbing liquid is small.
As a method for separating and removing the zero-valent mercury, there is a method of bringing exhaust smoke containing a sulfurous acid gas and mercury into contact with an absorbing liquid containing persulfuric acid and an iodine compound (see Patent Literature 1 or the like). In addition, there is also a method of adding an iodine gas into an exhaust gas containing a sulfurous acid gas and mercury and then bringing this mixture into contact with an absorbing liquid in a wet method exhaust smoke desulfurization apparatus, in which an iodine compound is used as an iodine gas generating source (see Patent Literature 2 or the like). In such methods of separating and removing the zero-valent mercury by using the iodine compound, a problem that the use amount of the iodine compound is large is caused, and so there is a demand for reducing the amount of the iodine compound used.
In addition, there is a method of bringing exhaust smoke containing zero-valent mercury, a sulfurous acid gas, oxygen and water into contact with a mercury adsorbent obtained by causing an iodine compound or the like to be carried on a surface of a carbon-based material and being subjected to a water-repellent treatment (see Patent Literature 3). Even in the technique of Patent Literature 3, it is considered that an absorbing liquid to which an iodine compound is added is used to separate and remove the zero-valent mercury. However, there is a demand for reducing the use amount of the iodine compound used even in this case.
Incidentally, there is the demand for reducing the use amount of the iodine compound upon the separation and removal of the zero-valent mercury not only in the treatment of the above-described combustion exhaust gas containing sulfurous acid and zero-valent mercury but also in other gasses containing zero-valent mercury.